Permanent holographic recording medium

ABSTRACT

A holographic recording medium comprising a conductive substrate, a photoconductive layer and a room temperature vulcanizable silicone elastomer layer, which elastomer crosslinks to a thermoset condition upon application of an electric field from a corona discharge.

I United States Patent [1 1 [111 3,861,914

Gange Jan. 21, 1975 [54] PERMANENT HOLOGRAPIIIC RECORDING 3,716,3592/1973 Sheridon i. 96/15 MEDIUM [75] Inventor: Robert Allen Gange, BelleMead, Primary EXami'1erDa\/id Klein N J Assistant Examiner-John L.Goodrow Attorney, Agent, or Firm-Glenn H. Bruestle; Birgit E. [73]Assrgnee: RCA Corporation, New York, NY. Morris [22] Filed: Jan. 15,1973 [21] Appl. No.: 323,747 [57] ABSTRACT A holographic recordingmedium comprising a con- U-S- .1, H ductive ubstrate a photoconductivclayer and a room temperature vulcanizable ilicone elastomer layer Fleldof Search H elastomer cross links to a thermoset ondition uponapplication of an electric field from a corona dis- [56] ReferencesCited charge UNITED STATES PATENTS 2/l97l Urbach 96/l.l

8 Claims, 2 Drawing Figures PATENTEBJANZI 191s lie. I

PERMANENT IIOLOGRAPHIC RECORDING MEDIUM The invention described hereinwas made in the performance of work under a NASA contract and is subjectto the provision of section 305 of the National Aeronautics and SpaceAct of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

This invention relates to a holographic recording medium. Moreparticularly, this invention relates to a recording medium which formspermanent phase holograms after exposure to a corona discharge.

BACKGROUND OF THE INVENTION Phase holograms can be formed on a heatsoftened thermoplastic surface which selectively deforms during exposureto an applied charge pattern, as has been disclosed by Urbach in U.S.Pat. No. 3,560,205. According to this system, a corona discharge deviceconnected to a suitable recording medium ionizes the air near thesurface of the thermoplastic, whereupon positive ions are depositeduniformly on the surface of the thermoplastic. This surface is nowexposed to an image by means of coherent light split into an object beamand a reference beam in known manner. The light interacts with aphotoconductor which causes a redistribution of the charge in the areaswhere it impinges on the thermoplastic. When the thermoplastic ischarged again, the electric field increases in the previouslyilluminated areas. The thermoplastic is then exposed to a temperaturesufficient to soften the surface which deforms according to the electricfield, becoming thinner or forming valleys in the areas of high fieldintensity. When cooled to room temperature, a hologram is recorded as athickness variation or relief pattern in the thermoplastic. This processproduces phase holograms of excellent quality and resolution.

Such holograms can be erased in the absence of exposure and coronadischarge by heating the thermoplastic above its softening point to atemperature sufficient to allow the surface tension of the thermoplasticto revert to its undeformed state.

The use of known thermoplastic recording media has several drawbackswhen a permanent holographic record is desired. The materials which areeasily deformed during recording often have low melting points, and aretacky, soft materials which are readily damaged in handling or bycontact with dust particles. Further, inadvertent exposure to elevatedtemperatures will, of course, erase the holograms as noted above. Folgeret al., in U.S. Pat. No. 3,565,978, have disclosed a method ofreplicating holograms to form permanent recordings, but this methodinvolves several steps and is cumbersome and expensive.

SUMMARY OF THE INVENTION I have found that by proper choice of therecording surface, deformation phase holograms can be recorded inpermanent form. Such recording surfaces deform to form the holographicimage pattern and simultaneously cross link to a thermoset conditionupon exposure to an electric field from a corona discharge.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view ofone embodiment of a recording medium of the invention and FIG. 2 is across-sectional view of another embodiment of a recording medium of theinvention.

DETAILED DESCRIPTION OF THE INVENTION The recording medium most suitablefor use in the present invention comprises a conductive substrate, aphotoconductive insulating layer over the conductive substrate and aroom temperature vulcanizable silicone resin over the photoconductivelayer which silicone cross-links in the presence of a corona discharge.

The conductive substrate is conventional and can be flexible or rigid.It can be made of a conductive metal. such as aluminum, brass, copperand the like; or of a non-conductive substrate coated with a thinconductive layer. Suitable non-conductive substrates include glass,quartz, plastics and the like, which can be coated with a conductor suchas tin oxide, copper iodide, indium oxide and the like. Preferably, theconductive substrate is transparent. Such substrates are well known andinclude glass coated with a thin transparent tin oxide or indium oxidefilm or like-coated polymeric films of polyethylene terephthalate,polycarbonates, acrylics, polyurethanes and the like.

The insulating photoconductor is also conventional and can be inorganic,such as a layer of armorphous selenium, or pigments such as cadmiumsulfide, cadmium selenide, zinc sulfide, zinc selenide, zinc oxide, leadoxide, lead sulfide, mercuric sulfide, antimony sulfide, mercuric oxide,indium trisulfide, titanium dioxide, arsenic sulfide, gallium selenide,lead iodide, lead selenide, lead telluride, gallium telluride, mercuricselenide, and the like. Alternatively, the photoconductor can beorganic, such as anthracene-3- benzylideneaminocarbazole,poly-N-vinylcarbazole, 2,5-bis(p-aminophenyl-l )-l ,3,4-oxadiazole, 1,4-dicyanonaphthalene, 2,4-diphenylquinazolin,lmethyl-2-(3,4-dihydroxymethylenephenol)benzimidazole and the like.These and other organic photoconductors can be complexed with Lewisacids, such as 2,4,7-trinitrofiuorenone, as is known. High sensitivityphotoconductors such as poly-N-vinylcarbazole containing2,4,7-trinitrofluorenone are preferred.

The room temperature vulcanizable (RTV) silicones useful in therecording layer are elastomers which cross-link to a thermoset conditionupon exposure to a corona discharge. The preferred silicones aretransparent, colorless resins of dimethyl silicones or dimethyldiphenylsilicones. Suitable commercial products include RTV 619A and RTV 602silicones of General Electric, which require the addition of a curingagent; and 236 dispersions of Dow Corning Corporation and SFl 154silicone of General Electric which do not require the addition of acuring agent. Resins having a very low viscosity can be employed as is,such as the SFll54 resin. Higher viscosity resins can be diluted with asuitable organic solvent.

An alternate recording medium can be provided whereby a silicone resinas described above contains a photoconductive material dissolvedtherein. This composition can be applied directly to the conductivesubstrate. Soluble photoconductors include polyhalogenated aliphatichydrocarbons such as iodoform, carbon tetrabromide, methylene iodide,tetraiodoethylene and the like.

The silicone resins can be applied in any conventional manner, such asby spraying, dip coating, brushing and the like but is preferablyapplied by spin coating. The silicone layer must be thick enough so thatwell defined holographic patterns may be formed in it upon exposure, butif the silicone layer is too thick, exposure will not extend through theentire layer, leaving a soft, gel-like portion adjacent to thephotoconductor layer. The preferred thickness is about 1 micron, butoptimum thickness can vary depending on the strength of the coronadischarge.

In the case where a reactive curing agent must be added to the siliconeresin to effect curing, the resultant mixture may adversely affect thephotoconductive layer and a barrier layer may be required between thephotoconductive layer and the recording layer. A thin layer of atransparent polymeric material can be applied over the photoconductivelayer. An acrylic resin commercially availabel as Elvacite 20l3 resinfrom du Pont de Nemours & Company has been found to be excellent forthis purpose. This resin is a low molecular weightmethyl/n-butylmethacrylate copolymer having an inherent viscosity of 0.2determined from a solution containing 0.25 gram of polymer in 50 ml ofchloroform at 25C.

A defect free, thin film of the barrier polymer can be applied bydipping the photoconductor coated substrate into a solution of theacrylic resin or by spin coating in known manner. The thickness of thebarrier layer, although not critical, should be regulated so that anexcessive amount of applied electric field will not be lost across thislayer. Suitably, the barrier layer can be up to about 0.25 micron inthickness.

Alternatively, a cross-linked polystyrene layer applied by glowdischarge technique can be employed as the barrier layer.

Referring now to the drawings, in FIG. 1 there is shown across-sectional view of a recording medium of the invention. Therecording medium comprises a conductive layer 1 having a photoconductivelayer 2 thereon and a cross-linkable silicone layer 3 over the layer 2.

FIG. 2 is a cross-sectional view of another embodiment of a recordingmedium. This recording medium has a conductive layer 4, aphotoconductive layer 5 thereon, a thin barrier layer 6 on the layer 5and a cross-linkable silicone layer 7 on the barrier layer 6.

The recording media described herein have high write sensitivity andhigh readout efficiency for the storage of information useful whenpermanent holographic information is to be recorded. For example, therecording media can be employed to record identifying information forsecurity purposes in permanent form. The recording media can, of course,be coated with a protective finish to prevent abrasion with consequentdistortion or loss of the recorded holographic information if desired.

The invention will be further illustrated by the following examples butit is to be understood that the invention is not meant to be limited tothe details described therein.

ln the examples, parts and percentages are by weight unless otherwisenoted.

EXAMPLE 1 A glass substrate coated with a thin, transparent indium oxidelayer was dipped into a solution of poly-N- vinylcarbazole:trinitrofluorenone (10:1) in 1:1 pdioxanemethylene chloride so as toapply a layer about 1-2 microns thick.

Another solution was prepared containing 0.2 part of Elvacite 2013 in 5parts by volume of acetone warmed to 60C. to which 50 parts by volume ofwarm ethanol was then added. The coated glass substrate was immersed inthe above solution and withdrawn at a rate of 2 inches/sec. A uniformbarrier layer about 1,000A thick was deposited onto the photoconductivelayer.

A third solution was prepared by mixing 3.14 parts of a room temperaturevulcanizable silicone resin commercially available as RTV 619A with 0.31part of hardener for the silicone, and immediately adding 40 parts byvolume of n-hexane. This solution was spin coated at 1,800 rpm using aPreco whirler. The resultant layer was about 1 micron in thickness.

Holograms were formed in the recording medium prepared as abovefollowing the general procedure of U.S. Pat. No. 3,560,205, with ahelium-neon laser using a continuous corona discharge. The recordingmedium cross linked during discharge to form permanent holograms aboutone half micron deep.

EXAMPLE 2 The procedure of Example 1 was followed except using adifferent silicone layer as follows: 2.5 parts of a silicone resinavailable commercially as RTV 602 was mixed with 0.25 part of catalystand then 50 parts by volume of n-hexane immediately stirred in. Theresultant layer applied to the barrier layer was about 1 micron inthickness.

Permanent holograms were formed in the exposed recording medium.

EXAMPLE 3 The procedure of Example 1 was followed except that thebarrier layer was omitted and a different silicone layer was applied asfollows: a mixture was prepared containing 1 part by volume of siliconeresin 236 dispersion commercially available from Dow Corning Company and5 parts by volumeof n-hexane. This mixture was spun onto the coatedsubstrate at 1,000 rpm to form a layer about 1 micron thick.

Permanent holograms were formed in the exposed recording medium.

EXAMPLE 4 The procedure of Example 3 was followed except that adifferent silicone layer was applied as follows: a dimethyldiphenylsilicone fluid commercially available as SF-1154 from General Electricwas applied dropwise to a coated substrate so as to form a layer about 1micron thick.

A high quality permanent hologram was formed in the exposed recordingmedium.

I claim:

1. A medium for recording permanent phase holograms in the form of asurface relief pattern comprising in sequence an electrically conductivesubstrate, a photoconductive layer and an electrically alterable storagelayer, wherein the storage layer comprises a room temperaturevulcanizable silicone elastomer which crosslinks upon application of acorona discharge to a thermoset condition.

2. A medium according to claim 1 wherein the silicone layer is about 1micron thick.

3. A medium according to claim 1 wherein the conductive substrate istransparent.

4. A medium according to claim 1 wherein the photoconductive layer ispoly-N-vinyl carbazole.

8. A medium for recording permanent phase holograms in the form of asurface relief pattern comprising in sequence an electrically conductivesubstrate and a recording layer, the recording layer comprising a roomtemperature vulcanizable silicone elastomer containing dissolved thereina polyhalogenated aliphatic hydrocarbon photoconductor which cross-linksupon application of a corona discharge to a thermoset condition.

2. A medium according to claim 1 wherein the silicone layer is about 1micron thick.
 3. A medium according to claim 1 wherein the conductivesubstrate is transparent.
 4. A medium according to claim 1 wherein thephotoconductive layer is poly-N-vinyl carbazole.
 5. A medium accordingto claim 1 wherein a transparent barrier layer is interposed between thephotoconductive layer and the silicone elastomer layer.
 6. A mediumaccording to claim 5 wherein the barrier layer is an acrylic resin up toabout 0.25 micron in thickness.
 7. A medium according to claim 6 whereinthe acrylic resin is methyl/n-butylmethacrylate copolymer having aninherent viscosity of 0.2.
 8. A medium for recording permanent phaseholograms in the form of a surface relief pattern comprising in sequencean electrically conductive substrate and a recording layer, therecording layer comprising a room temperature vulcanizable siliconeelastomer containing dissolved therein a polyhalogenated aliphatichydrocarbon photoconductor which cross-links upon application of acorona discharge to a thermoset condition.